Hi Annette,
Welcome to Auctiva!
I've never been in the position of having to sell 600 unique items at a time on eBay, but I can explain how the process of importing listings from your Closed Listings page to your Saved Listings page is supposed to work. If you want to "relist" an item through Auctiva, you can just check the box next to the item on your Auctiva Closed Listings page and click the blue "Relist" button.
If you would like to move the listing from your Closed Listings page to your Saved Listings page, on the other hand, you would want to check the boxes next to the desired items on your Closed Listings page, and click the "Import Listings" button. This should move them over to your Saved Listings page, from where they can be revised/reposted within a few minutes.
Auctiva Checkout can only be applied to listings that are posted through Auctiva so, if you will be using Auctiva Checkout, I recommend doing your relisting through Auctiva. If an Auctiva Checkout item were to come to a close and you were to relist that item through eBay, the resulting relist would use eBay Checkout instead.
If you have any trouble with the importing process, or if you need further assistance from our team, I recommend contacting customer support using the form on the following page: http://www.auctiva.com/help/requesthelp.aspx
-Mike